The need for automatically communicating from a plurality of instrumentation devices located at fairly remote and inaccessible locations to a central collection center has been acutely present for many years. Techniques have been implemented that utilize the telephone or power lines of the subscriber or commodity user (i.e., the utility customer). The installation of such systems requires a hard wire connection be established between the instrument link and the user's phone or power lines. Such a connection can increase installation costs for example, due to trenching and the actions to restore the affected landscape and/or structures to their original condition.
Another solution for automated instrumentation reading employs an radio frequency (RF) communication link between the instrument links, the instrument reading unit, and a remote station for data collection. Typically the remote station transmits interrogation signals or messages to the instrument links, and the instrument links in response transmit an RF reply signal containing the requested information and an instrument link identifier. Examples of such RF communication link systems are disclosed in U.S. Pat. Nos. 4,031,513; 4,614,945; and 4,799,059.
The disclosed systems are subject to a number of constraints. Because of the reduced power levels for unlicensed operation, the instrument links have operating ranges for transmitting RF signals of about 1000 feet. This limitation in operating range results because FCC regulations limit the power output of unlicensed transmitters to a maximum field strength of 50 millivolts at 3 meters which corresponds to a transmitter power output of less than 1 milliwatt.
These systems are susceptible to RF interference because of the narrow band modulation schemes implemented. To counter this the disclosed systems typically transmit the same message repeatedly at shifted frequencies presuming that at least one of the transmitted messages would get through and be properly received. While such operation may be workable up to now, the FCC has recently allocated some of the bands being used to unlicensed spread spectrum operation. Spread spectrum operation in these bands can increase the noise level to the point that acceptable operation of such systems will be difficult or impossible.
The remote station are bulky and in general require that the remote station be mounted in a vehicle which transits the area with instrument links to be interrogated.
One system operates in a common wakeup mode, where an interrogation signal is sent to all instrument links within range of the remote station's transmitter. The problems with signal collision (i.e., simultaneous receipt of two instrument link reply signals) prevents reliable use in areas densely populated with instruments thereby restricting operation to rural and medium density instrument link environments. This problem increases if the transmission range of the instrument link is increased.
Therefore, it is an object of the present invention to provide a multi-mode remote instrument reading and telemetry system that offers high immunity to RF interference yet does not require FCC licensing of the instrument link transmitter.
It is another object of the present invention to provide a meter reading and telemetry system that can dynamically adjust processing gain to reflect environmental conditions and to achieve an optimal balance between data retrieval speed and collision resistance.
It is a further object of the present invention to provide a meter reading unit where the transmission range is significantly increased over that of known prior art systems.
It is yet another object of the present invention to provide a meter reading unit which uses power conservation techniques to extend the life of the power supply.